Question: $ B = \left[\begin{array}{rr}5 & -2 \\ -1 & 5\end{array}\right]$ $ A = \left[\begin{array}{rr}2 & 0 \\ 0 & 2\end{array}\right]$ What is $ B A$ ?
Answer: Because $ B$ has dimensions $(2\times2)$ and $ A$ has dimensions $(2\times2)$ , the answer matrix will have dimensions $(2\times2)$ $ B A = \left[\begin{array}{rr}{5} & {-2} \\ {-1} & {5}\end{array}\right] \left[\begin{array}{rr}{2} & \color{#DF0030}{0} \\ {0} & \color{#DF0030}{2}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ B$ , with the corresponding elements in column $j$ of the second matrix, $ A$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ B$ with the first element in ${\text{column }1}$ of $ A$ , then multiply the second element in ${\text{row }1}$ of $ B$ with the second element in ${\text{column }1}$ of $ A$ , and so on. Add the products together. $ \left[\begin{array}{rr}{5}\cdot{2}+{-2}\cdot{0} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ B$ with the corresponding elements in ${\text{column }1}$ of $ A$ and add the products together. $ \left[\begin{array}{rr}{5}\cdot{2}+{-2}\cdot{0} & ? \\ {-1}\cdot{2}+{5}\cdot{0} & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ B$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ A$ and add the products together. $ \left[\begin{array}{rr}{5}\cdot{2}+{-2}\cdot{0} & {5}\cdot\color{#DF0030}{0}+{-2}\cdot\color{#DF0030}{2} \\ {-1}\cdot{2}+{5}\cdot{0} & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{5}\cdot{2}+{-2}\cdot{0} & {5}\cdot\color{#DF0030}{0}+{-2}\cdot\color{#DF0030}{2} \\ {-1}\cdot{2}+{5}\cdot{0} & {-1}\cdot\color{#DF0030}{0}+{5}\cdot\color{#DF0030}{2}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}10 & -4 \\ -2 & 10\end{array}\right] $